According to an embodiment, a photon detector includes a photoelectric converter, a voltage supplier, a potential variation detector, and an adjustor. The photoelectric converter performs photoelectric conversion based on an incident photon and outputs current corresponding to number of photons. The voltage supplier supplies a voltage to the photoelectric converter. The potential variation detector detects potential variation of a cathode of the photoelectric converter. The adjustor adjusts a potential of an anode of the photoelectric converter based on the potential variation detected by the potential variation detector so that variation in a potential difference between the cathode and the anode of the photoelectric converter is reduced, and outputs current from the photoelectric converter to the outside.